Indium gallium nitride smoothing structures for III-nitride devices

ABSTRACT

A smoothing structure containing indium is formed between the substrate and the active region of a III-nitride light emitting device to improve the surface characteristics of the device layers. In some embodiments, the smoothing structure is a single layer, separated from the active region by a spacer layer which typically does not contain indium. The smoothing layer contains a composition of indium lower than the active region, and is typically deposited at a higher temperature than the active region. The spacer layer is typically deposited while reducing the temperature in the reactor from the smoothing layer deposition temperature to the active region deposition temperature. In other embodiments, a graded smoothing region is used to improve the surface characteristics. The smoothing region may have a graded composition, graded dopant concentration, or both.

BACKGROUND

Semiconductor light-emitting diodes (LEDs) are among the most efficientlight sources currently available. Materials systems currently ofinterest in the manufacture of high-brightness LEDs capable of operationacross the visible spectrum are Group Ill-V semiconductors, particularlybinary, ternary, and quaternary alloys of gallium, aluminum, indium, andnitrogen, also referred to as III-nitride materials. Typically,III-nitride layers are epitaxially grown on sapphire, silicon carbide,or gallium nitride substrates. Sapphire substrates are often used,despite their poor structural and thermal match with III-nitride layers,because of sapphire's wide availability, hexagonal symmetry, and ease ofhandling and pregrowth cleaning. See, for example, S. Strite and H.Morkoc, GaN, AlN, and InN: A review, J. Vac. Sci. Technol. B 10(4),July/August 1992, p. 1237.

To ensure LEDs with good performance, e.g., high brightness, highefficiency, or high reliability devices, the properties of layerinterfaces must be carefully considered. Of particular interest are thelayer interfaces below and within the active region. The quality oflayer interfaces is controlled by the condition of the growth surface onwhich successive layers are deposited. Among conditions that lead topoor growth surface quality are substrate surface cleanliness, substratesurface misorientation, poor growth conditions, and impurities.

One method to achieve smooth GaN surface morphology is to grow a thicklayer of GaN at high temperature (approximately 1100° C.) and highgroup-V-to-group-III molar gas phase concentration ratios. GaN layersgrown in such a manner have a high lateral-to-vertical growth rate ratiocompared with GaN layers grown under standard growth conditions,allowing the GaN layers to overgrow rough surfaces and provide a smoothsurface for the growth of subsequent device layers grown on the GaNlayer. However, in order to achieve a smooth, planar surface, GaN layersgrown in this manner must be thick and require a long growth time.Further, In-containing active regions in an LED or laser diode mayrequire surface smoothness conditions that differ from conditions thatcan be provided by the above-described method.

SUMMARY

In accordance with the invention, a smoothing structure containingindium to prepare for active region growth is formed between thesubstrate and the active region of a III-nitride light emitting device.In some embodiments, the smoothing structure is separated from theactive region by a spacer layer, and the smoothing structure is at least200 angstroms thick and within 0.5 microns of the active region. In afirst embodiment, the smoothing structure is a single layer that is moreheavily doped than the spacer layer. In a second embodiment, the spacerlayer is thinner than a barrier layer in the active region of thedevice. In a third embodiment, a smoothing layer is used in combinationwith a misoriented substrate. The smoothing structures of the presentinvention may improve the surface characteristics of the layers grownover the smoothing structure, particularly the active region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an LED with a smoothing layer and a spacer layer inaccordance with the present invention.

FIG. 2 illustrates an LED with the n-contact formed on the smoothinglayer.

FIG. 3 illustrates the conduction band edge energy of the layers of thedevice illustrated in FIG. 1.

FIG. 4 illustrates the active region, spacer layer, and smoothing layerof one embodiment of the invention.

FIG. 5 illustrates an LED with a graded smoothing region.

FIG. 6A illustrates the indium composition of devices with and withoutcompositional grading in the smoothing region.

FIG. 6B illustrates the dopant concentration of devices with and withoutdopant concentration grading in the smoothing region.

FIG. 6C illustrates six examples of grading profiles.

FIG. 7 illustrates a compositional superlattice smoothing structure.

FIG. 8 illustrates an example of the n-type region of FIGS. 1, 2 and 5.

FIG. 9 illustrates the relative external quantum efficiency of deviceswith and without a smoothing layer.

FIG. 10 illustrates the relative external quantum efficiency of deviceswith and without a smoothing layer.

FIGS. 11A and 11B illustrate atomic force microscope surface micrographsof III-nitride LED multiple quantum well active regions.

FIG. 12 illustrates a display device incorporating LEDs according toembodiments of the present invention.

FIG. 13 illustrates the relative external quantum efficiency of deviceswith a smoothing layer combined with an intentionally misorientedsubstrate.

DETAILED DESCRIPTION

According to the present invention, a smoothing structure containingindium is incorporated into a III-nitride device in order to growIII-nitride epitaxial layers with desirable surface characteristics. Insome embodiments, a spacer layer separates the smoothing structure fromthe active region. III-nitride semiconductor layers as used hereinrefers to compounds represented by the general formulaAl_(x)Ga_(y)In_(l−x−y)N(0≦x≦1, 0≦y≦1, 0≦x+y≦1), which may furthercontain group III elements such as boron and thallium and in which someof the nitrogen may be replaced by phosphorus, arsenic, antimony, orbismuth.

FIG. 1 illustrates a cross section of a III-nitride LED including asmoothing structure that is a single layer. An n-type region 12 isformed on a substrate 11 such as sapphire. The smoothing layer 14 isformed over n-type region 12. The smoothing layer is typically an n-typelayer located beneath the active region, when viewing the LED with thesubstrate as the lowest layer, within 5000 angstroms of the activeregion. The smoothing layer can have a thickness ranging from about 200angstroms to several microns. Smoothing layer 14 has a lower indiumcomposition than active region 16. Typically, smoothing layer 14 is anInGaN layer containing 2-12% indium. In a preferred embodiment, thesmoothing layer contains 2 to 6% indium.

A spacer layer 15 separates active region 16 from smoothing layer 14.Spacer layer 15 typically does not contain In and may be, for example,GaN or AlGaN. Active region 16 is typically a multiple quantum wellstructure of AlInGaN or InGaN, with an indium composition between 5 and50% and an aluminum composition between 0 and 50%. A p-type region 17 isformed over the active region. P-contact 19 is formed on the uppersurface of p-type region 17, and an n-contact 18 is formed on an exposedportion of n-type region 12. Alternatively, n-contact 18 is formed on anexposed portion of smoothing layer 14, as illustrated in FIG. 2.

FIG. 3 illustrates the relative position of the conduction bandedgeenergy of the layers of the device illustrated in FIG. 1. As illustratedin FIG. 3, since smoothing layer 14 contains indium, it has a bandgapthat is less than n-type region 12 and spacer layer 15. The bandgap ofsmoothing layer 14 is greater than the bandgap of active region 16. Thehigh bandgap and small thickness of spacer layer 15 minimizes absorptionin the spacer layer of light emitted from the active region. The higherbandgap of the smoothing layer 14 than the active region reducesabsorption of light emitted from the active region in the smoothinglayer.

In a first embodiment of the invention, the smoothing layer is moreheavily doped than the spacer layer. The smoothing layer is doped with,for example, Si to a concentration between 2e17 cm⁻³ and 2e19 cm⁻³. Inthe first embodiment, the spacer layer is n-type and doped with, forexample, Si to a concentration between undoped and 2e18 cm⁻³. In thefirst embodiment, the spacer layer has a thickness ranging from about 10angstroms to 1 micron, with a typical thickness of 150 to 200 angstroms.Growth of the spacer layer allows the growth conditions, e.g.temperature, to be adjusted from the growth conditions of the smoothinglayer to the growth conditions of the active region. The thickness ofthe spacer layer is optimized to be thick enough to stabilize growthconditions during fabrication for growth of the active region, and thinenough to not diminish the beneficial effects of the smoothing layer onthe surface characteristics of the semiconductor layers grown over thesmoothing layer.

The spacer layer of the first embodiment has a dopant concentrationlower than n-type region 12, thus the spacer layer is a more resistivelayer that can help to spread current evenly into the active region,preventing current from crowding in the shortest paths between then-contact and the p-contact. The thickness of the spacer layer isselected based on the dopant concentration in the spacer layer such thatthe spacer layer does not significantly add to the forward voltage ofthe device.

FIG. 4 illustrates the active region, spacer layer, and smoothing layerof a second embodiment of the invention. The active region 16 of thedevice is typically a multiple quantum well structure, with at least onebarrier layer 51 separating two or more well layers 50. Though fourwells and three barriers are illustrated, the active region can havemore or fewer well layers and barrier layers, or be a single quantumwell active region. In the second embodiment, the spacer layer 15 isthinner than the thickness of a barrier layer. Barrier layers 51 canrange in thickness from 25 angstroms to one micron, and are typicallyabout 100 to 150 angstroms thick. Thus, in the second embodiment, thespacer layer typically has a thickness ranging from about ten angstromsto about 150 angstroms. Forming a spacer layer that is thinner than thebarrier layers in the active region is beneficial because as the spacerlayer gets thicker, the smoothing layer underlying the spacer layer maybe less able to influence the surface characteristics of layers grownover the smoothing layer.

In a third embodiment, a smoothing structure is incorporated into adevice grown on a miscut substrate. Such devices may show a furtherimprovement in device performance. Miscut substrates are prepared suchthat the first surface upon which growth is initiated deviates inorientation by a small angle from a major crystallographic plane, forexample the (0001) c-plane of sapphire. Miscut substrates have been usedin several materials systems, including III-nitrides, for differentpurposes. However, in accordance with the present invention, thecombination of miscut substrates with a smoothing layer may result in alarger improvement in device performance than either taken alone, asillustrated in FIG. 13. The magnitude of the miscut is important whenthe combination of smoothing layers and miscut substrates isimplemented. In general, we anticipate that there is an optimal miscutangle, which depends on the smoothing layer thickness, composition, andthe dopant concentration of the n-type region. The optimal miscut anglemay also depend on growth conditions. In principle, the combination ofsmoothing layers with miscut substrates is effective for all substrates,including sapphire, silicon carbide, and GaN. Improved deviceperformance has been observed in devices grown on miscut sapphiresubstrates ranging from 0.2 to 2 degrees off the (0001) c-plane ofsapphire.

A fourth embodiment of the device is illustrated in FIG. 5. In thefourth embodiment, the smoothing structure is a graded smoothing region60. Graded smoothing region 60 may have a graded composition, such asindium composition or aluminum composition, a graded dopantconcentration, or both a graded composition and a graded dopantconcentration. Devices incorporating a graded smoothing region 60 may ormay not include a spacer layer of constant composition and constantdopant concentration between the graded smoothing region and the activeregion. In combination with a graded smoothing region, the spacer layermay be, for example, doped or undoped GaN, AlGaN, InGaN, or AlInGaN.Typically, some portion of the graded smoothing region contains indium.

As used herein, the term “graded smoothing region ” is meant toencompass any structure that achieves a change in composition and/ordopant concentration in any manner other than a single step incomposition and/or dopant concentration. In one example, the gradedsmoothing region is a stack of layers, each of the layers having adifferent composition and/or dopant concentration than either layeradjacent to it. If the layers are of resolvable thickness, the gradedsmoothing region is known as a step-graded or index-graded region. Inthe limit where the thickness of individual layers approaches zero, thegraded smoothing region is known as a continuously-graded region. Thelayers making up the graded smoothing region can be arranged to form avariety of profiles in composition and/or dopant concentration versusthickness, including, but not limited to, linear grades, parabolicgrades, and power-law grades. Also, graded smoothing region s are notlimited to a single grading profile, but may include portions withdifferent grading profiles and one or more portions with substantiallyconstant composition and/or dopant concentration regions.

FIG. 6A illustrates the indium composition of the layers of a devicewith no compositional grading and five devices having indium-compositiongrading in graded smoothing region 60 of FIG. 5. Device A is the deviceillustrated in FIGS. 1 and 2. In device A, n-type region 12 has noindium, smoothing layer 14 contains some indium, spacer layer 15 has noindium, and the active region has several indium rich well layers.

Devices B, C, and D each have a graded indium composition in thesmoothing region and a spacer layer of constant composition separatingthe graded smoothing region from the active region. In device B, n-typeregion 12 contains no indium. In graded smoothing region 60, the indiumcomposition is gradually increased through smoothing region 60. Theindium composition can be increased, for example, by graduallyincreasing the ratio of the flow rate of indium-containing precursorgases to the flow rate of gallium-containing precursor gases duringgrowth, or by gradually lowering the growth temperature while keepingthe ratio of the flow rates of indium- and gallium-containing precursorgases constant. In device C, the indium composition is first abruptlyincreased, then gradually decreased through graded smoothing region 60.The indium composition can be decreased, for example, by graduallydecreasing the ratio of the flow rate of indium-containing precursorgases to the flow rate of gallium-containing precursor gases duringgrowth, and/or by gradually raising the growth temperature. The indiumcomposition may vary in devices B and C, for example, from 0% to about12%. Spacer layer 15 is adjacent to the active region and containslittle or no indium. In device D, the indium composition is increased ina first portion of graded smoothing region 60, then held constantthrough a second portion of the graded smoothing region.

Devices E and F do not have a constant composition and dopantconcentration spacer layer adjacent to the active region. Devices thatdo not incorporate a spacer layer do not necessarily have thicker gradedsmoothing regions 60 than devices that do incorporate spacer layers. Indevice E, a lower portion of graded smoothing region 60 has a constantindium composition. The composition of indium in the upper portion ofthe smoothing region is then reduced, for example, from about 12% in thelower portion of the smoothing region to about 0% in the portion of thesmoothing region adjacent to the active region. As described above, thecomposition of indium in the graded smoothing region is reduced bydecreasing the ratio of the flow rate of indium-containing precursorgases to the flow rate of gallium-containing precursor gases duringgrowth and/or by raising the temperature during growth. In device F, afirst portion of the graded smoothing region has an increasingcomposition of indium, a second portion has a constant indiumcomposition, then a third portion has a decreasing composition ofindium.

The devices illustrated in FIG. 6A are merely examples of compositionalgrading possible in graded smoothing region 60, and are not intended torepresent all the ways that the composition in these layers may begraded in accordance with the invention. Many other compositionalgrading schemes are possible, as is apparent to one skilled in the art.For example, compositional grading need not be linear, it can be, forexample, parabolic. Further, any of the compositional grading schemesdescribed above can be implemented with or without spacer layers, andwith or without dopant concentration grading or composition grading ofgroup III elements other than indium.

Compositional grading as described above in reference to FIG. 6A mayoffer several advantages. Graded composition region s can be used toengineer the band structure of the device, both by grading band gap andpiezoelectric charge between adjacent layers. Graded composition regions also eliminate the need for growth interruptions to change processconditions between adjacent layers, since the process conditions such asreactor temperature and precursor gas flow rate can be adjustedgradually through the graded region. Growth interruptions may causeimpurity accumulation, crystal defect formation, and surface etching atthe interfaces between layers, so the removal of growth interruptionsvia graded regions not only simplifies the growth process, but improvesdevice performance by eliminating problems at interfaces that can reducecarrier confinement and efficiently trap carriers. The compositionallygraded smoothing region should be designed for minimal absorption oflight emitted by the active region. Preferably, the minimum bandgapenergy within the smoothing region should be larger than the photonenergy of light emitted from the active region.

In addition to or instead of grading composition, the dopantconcentration in smoothing region 60 may be graded. FIG. 6B illustratesa device with no dopant concentration grading and five devices withdopant concentration grading. Device A is a device such as thatdescribed in the first embodiment. N-type region 12 is highly doped,smoothing layer 14 is doped less than n-type region 12, and spacer layer15 is doped less than smoothing layer 14. Each of n-type region 12,smoothing layer 14, and spacer layer 15 have a substantially uniformdopant concentration.

In device B, n-type region 12 has a uniform dopant concentration, thenthe dopant concentration is gradually reduced through graded smoothingregion 60. Device B includes a spacer layer 15 of constant dopantconcentration between the graded smoothing region 60 and the activeregion (not shown). In devices C, D, E, and F, no spacer layer separatesgraded smoothing region 60 from the active region (not shown). In deviceC, the concentration of dopant is abruptly reduced, then graduallyincreased through graded smoothing region 60. In device D, theconcentration of dopant is first gradually reduced in a first portion ofthe smoothing region, then held constant through a second portion of thegraded smoothing region adjacent to the active region. In device E, thedopant concentration is first abruptly reduced, then held constant in afirst portion of the smoothing region, then gradually increased througha second portion of the smoothing region adjacent to the active region.In device F, the dopant concentration is gradually reduced in a firstportion of the smoothing region, then held constant in a second portionof the smoothing region, then gradually increased in a third portion ofthe smoothing region adjacent to the active region.

The devices illustrated in FIG. 6B are merely examples of dopantconcentration grading possible in graded smoothing region 60, and arenot intended to represent all the ways that the dopant concentration inthese layers may be graded in accordance with the invention. Forexample, the dopant concentration need not be linear as illustrated inFIG. 6B. Also, any of the dopant concentration grading schemes describedabove can be implemented with or without spacer layers, and with orwithout compositional grading. Grading the dopant concentration ingraded smoothing layer 60 may offer advantages, such as processsimplification or compensation of piezoelectric charge to reduce theforward voltage of the LED or laser diode.

FIG. 6C illustrates six examples of possible grading profiles. Thegrading profiles illustrated in FIGS. 6A and 6B need not be linear, asshown in grading profile A of FIG. 6C. The grading profiles can also benonlinear monotonic profiles, such as the parabolic profile illustratedin grading profile B or the stepped profile illustrated in gradingprofile C.

Alternatively, the grading profiles can be superlattice structures thatare not monotonic, as illustrated in grading profiles D, E, and F. Ingraded superlattices, the layers making up the graded smoothing regionalternate systematically in a fashion such that the moving averagecomposition and/or dopant concentration of the layers varies along thethickness of the graded smoothing region in a manner similar to thatdescribed above for non-superlattice regions. Superlattice gradingprofiles E and F are appropriate for composition grading, and profilesD, E, and F are appropriate for dopant concentration grading. Insuperlattices D, E, and F, sets of layers with different gradingprofiles alternate. The layer sets are arbitrarily named “1” and “2” ingrading profiles D, E, and F. In profile D, the dopant concentrationincreases in the first set of layers (labeled “1”). The layers in thefirst set alternate with layers in the second set of layers (labeled“2”) which have a constant dopant concentration. In profile E, thedopant concentration or indium composition increases in both the firstand second set of layers. However, the layers in the first set increaseover a different composition or dopant concentration range than thelayers in the second set, thus the overall structure is not monotonic.In profile F, the dopant concentration or indium composition decreasesin the first set of layers, and increases in the second set of layers.

In a fifth embodiment of the invention, the smoothing structure is acompositional superlattice, i.e. a stack of alternating thin layers ofGaN-based materials with different compositions. FIG. 7 illustrates asmoothing superlattice in accordance with the fifth embodiment. Thesuperlattice is made up of alternating layers 14 a and 14 b of highindium composition materials and low indium composition materials. HighIn composition layers 14 a are, for example, about 10 to about 30angstroms thick and have an indium composition between about 3 and 12%.Low In composition layers 14 b are, for example, about 30 to about 100angstroms thick and have an indium composition between about 0 and 6%.

In some embodiments, a device may incorporate several smoothing layersin order to achieve the desired surface smoothness. FIG. 8 illustratesone example of n-type region 12 of FIGS. 1, 2, and 5 in more detail.N-type region 12 can include a nucleation layer 12 a formed oversubstrate 11. An undoped GaN layer 12 b with a thickness of about 0.5 μmoverlays nucleation layer 12 a. Doped GaN layers 12 c and 12 d overlayundoped layer 12 b. layer 12 c is a moderately doped GaN layer with athickness of about 1 μm and an n-type dopant concentration of about 1e18cm⁻³. layer 12 d is a more heavily doped layer contact layer with athickness of about 2 μm and an n-type dopant concentration of about 1e19 cm⁻³. Additional indium-containing smoothing structures may bepositioned between layers 12 a and 12 b, between layers 12 b and 12 c,and between layers 12 c and 12 d, or within any of the layers. Inembodiments incorporating multiple smoothing layers at interfacesbetween layers or within the layers of the device, at least 100angstroms of III-nitride material preferably separates the smoothinglayers.

The device illustrated in FIG. 1 may be fabricated by first polishingsubstrate 11, such as SiC, sapphire, GaN, or any other suitablesubstrate, on one or both sides then preparing the substrate for growthwith various cleans. GaN-based semiconductor layers 12, 14, 15, 16, and17 are then epitaxially grown on substrate 11 through metal-organicchemical vapor deposition, molecular beam epitaxy, or another epitaxialtechnique. The substrate is placed in a reactor and precursor gases,such as tri-methyl gallium and ammonia, are introduced which react atthe surface of the substrate to form GaN. First, a III-nitridenucleation layer such as AlN, GaN, or InGaN may be grown over substrate11. N-type region 12 doped with, for example, Si, Ge, or O, is thenfabricated over the nucleation layer. N-type region 12 is typicallyformed at about 1050° C.

Smoothing layer 14 according to the first, second, or third embodimentmay be formed by, for example, removing the tri-methyl gallium from thereactor, then introducing tri-methyl indium, tri-ethyl gallium, andammonia into the reactor. Smoothing layer 14 is grown at a lowertemperature than the n-type region (grown at about 1050° C.) and at ahigher temperature than the active region (grown between 700 and 900°C.), for example 960° C. Formation of the smoothing layer at a highertemperature than the active region typically results in better surfacecharacteristics of the smoothing layer, and therefore of the layersgrown over the smoothing layer. After smoothing layer growth isfinished, the indium-containing gas is removed, and GaN or AlGaN spacerlayer 15 is formed. The first part of spacer layer 15 is fabricatedwhile lowering the temperature from the smoothing layer growthtemperature to the active region growth temperature, typically between700 and 900° C. The second part of spacer layer 15 is fabricated at theactive region growth temperature to stabilize the growth temperature foractive region growth.

Graded smoothing region s according to the fourth embodiment of theinvention are grown by altering the process conditions described above,as described in reference to FIGS. 6A and 6B. For example, anindium-composition graded smoothing region can be grown by graduallychanging the temperature and/or the ratio of the flow rates of theindium-containing precursor gas to gallium-containing precursor gasduring growth. A dopant concentration graded region can be grown bygradually changing the ratio of flow rate of a dopant-containing gas tothe flow rate of the group-III containing gases during growth.

When spacer layer 15 is finished growing, the flow rates of indium- andgallium-containing precursor gases are adjusted to form the well andbarrier layers of active region 16. The growth temperature affects howmuch indium is incorporated into a layer, though the composition of alayer can be also be controlled by other process conditions such as theratio of the flow rates of the indium- and gallium-containing precursorgas. Typically, the higher the temperature, the less indium isincorporated. Since smoothing layer 14 and active region 16 bothtypically contain indium, if the device did not include a spacer layer15, growth would have to be stopped after the formation of smoothinglayer 14 to allow the reactor to cool, in order to form the activeregion 16 with the proper composition of indium. Stopping growth inorder to allow the reactor to cool can allow impurity accumulation orsurface etching to occur at the surface of the smoothing layer, whichcan harm smoothing layer and subsequent layer surface characteristicsand device performance.

After the active region is formed, other precursor gases are addedand/or removed to form p-type region 17 of AlGaN or GaN doped with, forexample, Mg. P-type layers which may be optimized for conductivity orohmic contact formation may be formed within p-type region 17. Ap-metalization layer, which will later form the p-contact, is thendeposited over the semiconductor layers. The device is patterned andportions of the p-metalization layer, the p-type semiconductor layers,the active region, and the n-type semiconductor layers are etched awayto expose a portion of n-type region 12. An n-contact is then depositedon the exposed portion of n-type region 12. In another embodimentillustrated in FIG. 2, the etch does not penetrate into n-type region12, rather it exposes a portion of n-type smoothing layer 14. In thisembodiment, the n-contact is formed on smoothing layer 14. N- andp-contacts may be, for example, Au, Ni, Al, Pt, Co, Ag, Ti, Pd, Rh, Ru,Re, and W, or alloys thereof.

In accordance with the invention, devices incorporatingindium-containing smoothing structures may offer several advantages.First, the use of a smoothing structure can recover two-dimensionalstep-flow-type growth of smooth semiconductor surfaces, even afterundesirable three-dimensional island growth has begun. Three dimensionalisland growth can be caused by large substrate surface misorientation,poor surface preparation, or growth initiation steps, such as silicondosing, designed to reduce the density of crystal dislocations. Asdescribed above, surface morphology has an impact on device performance;thus smoothing structures may enhance both the efficiency and thereliability of III-nitride LEDs.

FIG. 9 illustrates the external quantum efficiency of LEDs grown onmisoriented substrates with large misorientation angle (e.g. 2 degreesfrom the c-plane) and grown with and without smoothing layers. GaN-basedsemiconductor layers are typically grown on the c plane (0001) ofsapphire, as GaN grown on (0001) sapphire exhibits superiorcrystallinity to GaN grown on other planes of sapphire. If theGaN-growth surface of the sapphire substrate is significantlymisoriented from (0001) and the device does not include a smoothinglayer, the surface characteristics of the epitaxially-grown GaN layersand device performance can suffer. FIG. 9 demonstrates the ability of asmoothing layer to eliminate poor device performance caused by growth onmisoriented substrates. The devices grown with a smoothing layer aremuch more efficient than the devices grown without a smoothing layer.

Second, the use of an InGaN smoothing layer beneath the active regionincreases the brightness of the resulting device. FIG. 10 illustratesthe relative efficiency of devices produced with and without InGaNsmoothing layers. The triangles represent devices with an InGaNsmoothing layer, and the circles represent devices without an InGaNsmoothing layer. As illustrated in FIG. 10 the use of an InGaN smoothinglayer produces a device that is about twice as efficient as a device ofthe same color without an InGaN smoothing layer.

The ability of smoothing layers to recover two dimensional growth afterthree dimensional growth has begun is demonstrated by FIGS. 11A and 11B,which illustrate the effect of smoothing layers on silicon dosing.Silicon dosing has been proposed as a method of reducing extendedstructural defects in GaN layers. During growth, the GaN layer isexposed to silane, which may deposit on the surface of the GaN as SiN.When growth is started again, the resulting GaN has fewer dislocations.However, silicon dosing can lead to three-dimensional island growth.FIGS. 11A and 11B are micrographs derived from Atomic Force Microscopymeasurements for LED multiple quantum well (MQW) structures. In bothcases, a surface below the active region was exposed to silicon dosing.FIG. 11B illustrates a MQW active region without a smoothing layer.Silicon-dosing has caused three-dimensional island growth which resultsin an extremely rough surface. FIG. 11A illustrates a similar MQWstructure, but with a smoothing layer between the Si-dosed layer and theMQW. The smoothing layer beneath the MQW has restored the smoothtwo-dimensional growth of low-temperature III-nitride. Thus, the benefitof reduced defect density as accomplished by the Si-dosing technique canbe utilized for LED structures, because the smoothing structuresaccording to the present invention may provide smooth interfaces. Theluminescence efficiency of two Si-dosed MQW structures is illustrated inTable 1.

TABLE 1 Si-dosing below Si-dosing, no MQW Structure smoothing layersmoothing layer Normalized Light 100% 7% Intensity Surface MorphologySmooth Islands

The first structure includes a smoothing layer grown on top of aSi-dosed surface. The first structure exhibits bright luminescence. Theintensity of the first structure was normalized to 100%. The secondstructure is Si-dosed and did not include the smoothing layer. Theluminescence efficiency is severely reduced.

Third, smoothing layers may expand the temperature range in whichIII-nitride layers with smooth surface morphologies can be grown.Conventionally, MOCVD growth of GaN or AlGaN is limited to a smalltemperature window, as high temperature typically results in undesirablehexagonal surface features, and low temperature results in the formationof pits. Finally, smoothing layers allow growth of smooth III-nitridelayers without greatly increasing the complexity of device fabrication.

Blue and green LEDs formed in accordance with the invention areparticularly suitable for color displays using red, green, and blue LEDsas the pixel elements. Such displays are well known and are representedby FIG. 12. A display panel has an array of red, green, and blue LEDs,respectively, that are selectively illuminated by well known circuitryto display an image. Only three pixels are shown in FIG. 12 forsimplicity. In one embodiment, each primary color is arranged incolumns. In other embodiments, the primary colors are arranged in otherpatterns, such as triangles. The LEDs may also be used for backlightingan LCD display. Additionally, blue- or UV-emitting LEDs formed inaccordance with the present invention may be used in combination withvarious phosphor materials to generate white light.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from theinvention in its broader aspects and, therefore, the appended claims areto encompass within their scope all such changes and modifications asfall within the true spirit and scope of this invention. For example,the smoothing structures of the present invention are not limited tolight emitting devices (e.g. LEDs and laser diodes) and may be appliedto photo detectors, electronic devices, bipolar transistors, and deviceswhere interface quality is critical, such as high-electron mobilityfield-effect transistors.

We claim:
 1. A III-nitride light emitting device comprising: asubstrate; an n-type region overlying the substrate; an active regionoverlying the n-type region; a smoothing layer containing indium, thesmoothing layer being at least 200 angstroms thick and located betweenthe substrate and the active region; and a spacer layer located betweenthe smoothing layer and the active region; wherein the smoothing layercontains a greater dopant concentration than the spacer layer and asurface of the smoothing layer is located less than one half of onemicron from a surface of the active region.
 2. The light emitting deviceof claim 1 further comprising an n-contact connected to the smoothinglayer.
 3. The light emitting device of claim 1 wherein the spacer layerhas an n-type dopant concentration ranging from undoped to about 2e18cm⁻³.
 4. The light emitting device of claim 1 wherein the smoothinglayer has an n-type dopant concentration ranging from about 2e17 cm⁻³ toabout 2e19 cm⁻³.
 5. The light emitting device of claim 1 wherein thesmoothing layer is InGaN having a composition of In between about 2 andabout 12 percent.
 6. The light emitting device of claim 1 wherein thesmoothing layer is doped with Si.
 7. The light emitting device of claim1 wherein the spacer layer has a thickness of about 10 angstroms toabout one half of one micron.
 8. The light emitting device of claim 1wherein the smoothing layer has a thickness of about 200 angstroms toabout 4 microns.
 9. The light emitting device of claim 1 wherein thesmoothing layer is adjacent to the spacer layer.
 10. The light emittingdevice of claim 1 wherein the spacer layer is GaN or AlGaN.
 11. AIII-nitride light emitting device comprising: a substrate; an n-typeregion overlying the substrate; an active region overlying the n-typeregion, the active region comprising at least one well layer and atleast one barrier layer; a smoothing layer containing indium, thesmoothing layer being at least 200 angstroms thick and located betweenthe substrate and the active region; and a spacer layer located betweenthe smoothing layer and the active region, wherein the spacer layer isthinner than a barrier layer; wherein a surface of the smoothing layeris located less than one half of one micron from a surface of the activeregion.
 12. The light emitting device of claim 11 further comprising ann-contact connected to the smoothing layer.
 13. The light emittingdevice of claim 11 wherein the spacer layer has an n-type dopantconcentration ranging from undoped to about 2e18 cm⁻³.
 14. The lightemitting device of claim 11 wherein the smoothing layer has an n-typedopant concentration ranging from about 2e17 cm⁻³ to about 2e19 cm⁻³.15. The light emitting device of claim 11 wherein the smoothing layer isInGaN having a composition of In between about 2 and about 12 percent.16. The light emitting device of claim 11 wherein the smoothing layer isdoped with Si.
 17. The light emitting device of claim 11 wherein thespacer layer has a thickness of about 10 angstroms to about one half ofone micron.
 18. The light emitting device of claim 11 wherein thesmoothing layer has a thickness of about 200 angstroms to about 4microns.
 19. The light emitting device of claim 11 wherein the smoothinglayer is adjacent to the spacer layer.
 20. The light emitting device ofclaim 11 wherein the spacer layer is GaN or AlGaN.
 21. A III-nitridelight emitting device comprising: a substrate having a growth surface;an n-type region overlying the growth surface of the substrate; anactive region overlying the n-type region; and a smoothing layercontaining indium, the smoothing layer being located between thesubstrate and the active region; wherein the growth surface ismisoriented from a crystallographic plane of the substrate.
 22. Thelight emitting device of claim 21 wherein the substrate is sapphire andthe crystallographic plane is c-plane.
 23. The light emitting device ofclaim 21 wherein the growth surface is misoriented from thecrystallographic plane by about 0.2° to about 2°.
 24. A display devicecomprising: at least one blue light emitting device; at least one greenlight emitting device; and at least one red light emitting device;wherein at least one of the blue light emitting device, green lightemitting device, and red light emitting device comprises: a substrate;an n-type region overlying the substrate; an active region overlying then-type region; a smoothing layer containing indium, the smoothing layerbeing located between the substrate and the active region; and a spacerlayer located between the smoothing layer and the active region; whereinthe smoothing layer contains a greater dopant concentration than thespacer layer and a surface of the first smoothing layer is located lessthan one half of one micron from a surface of the active region.